Haiku can be built with/for GCC 2 or GCC 4 and ship with libraries for the other. Some time after R1, the gcc2-hybrid releases will probably be flipped to gcc4-hybrid, and the gcc2 libs kept around for backwards-compatibility purposes only.

The current setup is really only used to the benefit of applications - the kernel interface is not ABI- or API-compatible, for the most part, with that of BeOS R5.